Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Statement of Comprehensive Income [Abstract]
Net income	$ 50.2	$ 58.4
Other comprehensive income, before tax:
Change in net unrealized gains (losses) during the period	70.7	(26.4)
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax	0.0	(0.2)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification and Tax	1.0	(17.4)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	1.0	3.5
Deferred tax (expense) benefit related to other comprehensive (loss) income	(18.3)	10.1
Other comprehensive income (loss), net of tax	54.4	(30.4)
Other comprehensive income (loss), before tax	72.7	(40.5)
Comprehensive income, net of tax	$ 104.6	$ 28.0